The present invention relates to steering knuckle and kingpin assemblies in general, and more particularly to steering kingpin and knuckle assemblies for heavy motor vehicles.
Steering knuckle and kingpin assemblies, used in heavy motor vehicles such as trucks and the like, consist generally of a circularly cylindrical kingpin fastened through a substantially vertical bore in opposite ends of the vehicle front axle, and of a wheel spindle knuckle pivotable relative to the kingpin. The knuckle has an integral upper yoke and an integral lower yoke that straddle the axle end and pivot relative to the kingpin ends projecting above and below the axle. Appropriate bushings are fixedly fitted in the yoke bores, and the bearing surfaces in engagement, namely the internal cylindrical surface of the yoke bushing and the corresponding cylindrical peripheral surface of the kingpin ends, are subjected to substantial and variable loads, vibrations and shock, which eventually cause excessive wear, requiring replacement of the bushings, or of the kingpin, or both, replacement kingpins and bushings being generally marketed in the form of a complete kit or set. It is not uncommon to replace heavy truck steering knuckle and kingpin assemblies several times during the useful life of the vehicle.
Because the road wheel mounted on the knuckle spindle supports a proportional portion of the weight of the vehicle, the kingpin is subjected to torque force in a substantially vertical plane through the intermediary of the knuckle yokes and bushings. The forces applied to the kingpin ends in opposite directions tend to cause engagement of a localized area of the peripheral surface at the top of the upper end portion of the kingpin with a corresponding localized area of the inner surface of the upper yoke bushing, and of a localized area of the surface of the bottom of the kingpin lower portion with a corresponding localized area of the inner surface of the lower yoke bushing. The bearing surfaces in mutual contact are not the designed cylindrical bearing surfaces of the kingpin ends and yoke bushings as, under load, there is only point contact or line contact between the bearing surfaces causing excessive compressive stress. The bearing surfaces are generally not of equal lengths. For example the edge of the kingpin ends may project beyond the end of the shackle yoke bushings or, alternatively, the edge of the bushings may project beyond the kingpin ends. When such conditions exist, the excessive compressive stress of the bearing surface areas in engagement is further aggravated by the edge effect resulting from the loads being transmitted from the bushing to the kingpin end peripheral surface, at the edge of the bushings in the structure wherein the kingpin ends project beyond the bushings, or at the edge of the kingpin ends in structures wherein the kingpin ends do not project beyond the bushings. The edge of the bearing member in engagement with the surface of the other bearing member causes an indentation of the surface of the other bearing member, and further elastic or plastic deformation of both members at the line of contact, thus adding a stress riser at the point or line of maximum loading of the bearing surface, with accompanying failure and piercing of the lubricant film, in turn causing rapid deterioration of the bearing surfaces and propagation of wear from abrasion and from contact corrosion.